More specifically, said laser source is of the type comprising:                an active element comprising an elongated bar, of generally circular, but not exclusively circular, transverse section, comprising a doped matrix able to absorb a pumping beam being propagated longitudinally to amplify a laser radiation also being propagated longitudinally;        a pumping system, comprising pumping (laser) diodes able to emit said longitudinal pumping beam;        an optical transport system for directing the pumping beam emitted by said pumping system in said active element so as to obtain a longitudinal pumping; and        an optical cavity making it possible to extract said laser radiation.        
It is known that, to be effective, the pumping beam must be spectrally tuned to the absorption spectrum of the active element so that said pumping beam is absorbed and transfers its energy to the ion (rare earth or transition metal for example) doping said active element.
It is also known that the pumping (laser) diodes present an emission spectrum, normally a few nanometers wide, which is offset by 0.25 to 0.3 nanometers per degree Celsius, when the temperature of said pumping diodes is varied.
To ensure a satisfactory conformity of the wavelength of the pumping beam (obtained from said pumping diodes) with the absorption spectrum of the active medium, the art of mounting said diodes on Peltier modules is known, the function of which is to stabilize their temperature to better than 0.5° C., such that a centering of the wavelength is ensured to at least 0.2 nm.
However, notably in the context of military applications, the parameters of compactness, energy consumption and speed of deployment are of particular importance. Thus, the use of Peltier modules, which induces a high energy consumption and which necessitates a stabilization time of around one minute, represents a brake on the use of diode-pumped laser sources in compact systems. The same applies for other active diode temperature stabilization systems. Thus, the technology still used today, for example for terrestrial laser designators, is a flash pumping technology, which is not very cost effective and bulky.
To try to overcome this problem, it is therefore appropriate:                either to increase the tolerance of the active element to the wavelength drift, which is proposed for example by patent FR-2 803 697, for which the pumping beam is guided to pass several times through the active element;        or to apply a passive stabilization of the pumping diode emission wavelength, as proposed, for example, in the patent application US-2005/0018743 which describes the use of a system including one or more Volume Bragg Gratings (VBG) in order to condition one or more laser emission characteristics.        
However, the above solutions only make it possible to obtain an insensitivity of 3 to 10 nanometers corresponding to a diode temperature drift of 15 to 40° C. Such a range of thermal insensitivity falls far short of what is needed to use the pumping system, for example in a terrestrial laser designator, between −40° C. and +70° C.
The object of the present invention is to provide an active element and a laser source making it possible to obtain a thermal insensitivity of the laser emission over more than 15 nanometers.
It is known that the proportion of pumping energy absorbed by the active element depends, on the one hand, on the absorption coefficient α(λ) of the active element and, on the other hand, on the length of material L passed through by the pumping beam. This absorbed energy proportion Abs satisfies the relation Abs(λ)=1−Exp[−α(λ)L] in the case of a uniform material or Abs(λ)=1−Exp[−α1(λ)L1−α2(λ)L2− . . . αn(λ)Ln] in the case of a material made up of n zones of absorption αi(λ) and of length Li and Abs(λ)=1−Exp[−∫α(λ, z)dz] in the most general case where the doping and the absorption vary in the bar according to the longitudinal position z, λ designating the wavelength of the laser emission. Thus, to optimize said proportion Abs, it is appropriate to maximize, on the one hand, said absorption coefficient α for all the interesting wavelengths and, on the other hand, said length passed through by the pumping beam. In order for the proportion Abs to always remain greater than approximately 80-90% for all of the targeted spectral range, the absorption length must be adapted to the lowest coefficient.
It is known, moreover, that it is difficult to adequately extract the energy from a large volume of active element, in which the pumping energy would be dispersed. Thus, it is advantageous to set up a longitudinal pumping configuration, for which the absorption length of the pumping beam can be long, provided that the latter is colinear (or almost colinear) to the axis of the laser source. The active element is therefore then suitable for receiving and conveying a pumping radiation being propagated colinearly (or almost colinearly) to the axis of the laser source.
The main difficulty with longitudinal pumping at high power levels (greater than 500 W), if it is a triggered laser that is required, lies in the production of spurious effects, such as a spontaneous emission amplification (ASE amplification hereinbelow) or spurious emission modes (MEP modes hereinbelow). The ASE amplification comes from a spontaneous radiation, naturally emitted by the ions excited by the pumping beam and amplified by the gain resulting from the presence of these excited ions. As for the MEP modes, they come from the combination:                of reflections present at the edges of the active element and/or on any other reflector; and        of the laser gain originating from the excited ions.        
The combination of these two factors generates a spurious laser emission along one or more axes which are usually different from the main laser axis.
The ASE amplification is a parameter governed mainly by the gain and the maximum possible gain length in the active element. The only way to reduce its effect is to limit the gain length or the gain value.
Furthermore, the MEP modes are governed by the gain and the presence of spurious reflections which return photons to the laser so allowing gain cycling of these photons.